Volvo Penta has a remarkable history when it comes to marine power. The
company, which began in 1868 as a foundry specializing in cast-iron goods
and hardware, made its entry into marine engines in 1907 with a paraffin-fired
test engine dubbed the B1. In 1922 it introduced the U2, a two-cylinder,
two-stroke outboard, and in 1954, the world’s first series-manufactured
turbodiesel joined its growing line of engines, along with the MD1, the
world’s smallest direct-injection diesel. Two years later Volvo Penta
introduced the first charge-air-cooled marine diesel.

Lately this Swedish industrial giant has concentrated on developing a
series of engines incorporating the latest electronic technology that
could deliver power and torque where they are needed most. Complicating
the project were the all too-familiar concerns of safety, performance,
fuel efficiency, and of late, emissions. That program has now culminated
the new D-12s, four diesels rated at 615, 650, 675, and 700 hp.

The D-12 series is based on a 12.1-liter, turbocharged and aftercooled
in-line six featuring one electronic unit injector for each cylinder.
Its induction-hardened crankshaft has seven main bearings for durability
and two vibration dampers designed to reduce the roughness inherent in
a diesel engine. A freshwater aftercooler increases horsepower while minimizing
corrosion, and a large expansion tank is designed to accommodate optional
keel cooling.

Because the unit injectors develop very high injection pressures, fuel
is well atomized and dispersed when it enters the combustion chamber,
creating more complete combustion. Little fuel is returned to the tank,
which prevents warming of the tank’s fuel. Cooler fuel results in
more horsepower.

Making sure all that horsepower is used efficiently by the propulsion
system was a major concern of Volvo Penta engineers. “We designed
this engine to develop power and torque at the engine speed where it is
most needed,” says Urban Larsson, technical manager of the D-12 project.
“No power or fuel is wasted on torque that the propeller cannot handle
or power at the wrong rpm.” To understand how the engineers accomplished
that, you need to understand something of the nature of horsepower and
torque aboard a boat.